Back at the turn of this century, Melissa McCarthy came to the attention of millions of dedicated fans of the TV show “Gilmore Girls,” a small-town dramedy in which she played Sookie St. James, the cute, stylishly zaftig best friend of cute, stylishly un-zaftig Lorelei Gilmore. The show, an addictive combination of rapid-fire dialogue, of-the-moment pop references and shamelessly Capra-esque escapism, was fun to watch largely thanks to characters like Sookie, who was intelligent and warm and funny and, even in her most idealized moments, always recognizably human.

“Gilmore Girls” ended in 2007; four years later, McCarthy co-starred in the breakout comedy hit “Bridesmaids,” in which she played an abrasive, socially awkward misfit with a tomboyish demeanor and an aggressive libidinous streak. The movie ignited McCarthy’s career and, with the exception of the bland TV sitcom “Mike & Molly,” she’s been playing an iteration of her “Bridesmaids” persona ever since: In “Identity Thief,” she portrayed a mouthy sociopath on the lam with Jason Bateman and in “The Heat,” another smash success, she starred opposite Sandra Bullock as an explosively hostile Boston cop.

McCarthy goes back to the bilious well one more time in “Tammy,” a movie she co-wrote with her husband, Ben Falcone (who also directs), so she literally has only herself to blame. Her skin an angry shade of sunburned red, her hair a frizzled mass of bleached split ends, she blusters and bullies her way through a misbegotten movie that starts badly and ends worse. Tammy, McCarthy’s character, is a graceless, obnoxious, thoroughly unpleasant young woman who gets fired from her fast-food job and returns home to discover that her husband is having an affair; this sends her on a road trip to Niagara Falls with her grandmother, who, for reasons known only to heaven above and the good people of United Talent Agency, is played by Susan Sarandon, decked out in a curly gray wig and prosthetically swollen ankles.

The joke, if you can call it that, is that Granny Pearl is just as down-and-dirty as Tammy, and more sexually successful to boot. Most of “Tammy” consists of the two women getting entangled in some kind of un-funny, ill-advised encounter, which ends with a string of expletives and one or both of them knocking something over, whether it’s a suntan lotion display on a personal watercraft vendor’s counter or the salt and pepper shakers on the table of two men who spurn Tammy’s boorish flirtations. And . . .scene.

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If Sarandon thought “Tammy” might be a clever send-up of her role in the era-defining “Thelma & Louise,” she was sorely mistaken: She flounders and flails as she tries to find a movie to share with McCarthy, who does nothing smart or subversive with her character other than make her progressively more shallow and mean-spirited. Crude, coarse, clueless and supremely sure of her own surpassing intelligence and talent, Tammy is a cross between a toddler and a raging, human Id: She’s a tIddler. And Pearl’s a tippler, her alcoholism escalating from a morning beer to a full-on binge that ends with a cruel public joke about Tammy’s weight.

That scene, set at a sumptuously appointed lesbian Fourth of July party, features the movie’s most bracing exchange, which comes by way of the wonderful Kathy Bates, playing Pearl’s fabulously successful cousin Lenore. But as a brief moment of truth, it quickly gets lost within a tiresome, offensively dishonest movie whose happy ending feels gratuitous and tacked on: Lessons aren’t learned as much as hastily inserted directly from a Screenwriting 101 handbook.

“Tammy” is a bummer, not least because McCarthy’s fans know she’s better than this. We all miss Sookie, but we miss Melissa even more.